Eddy current probes are often used for non-destructive evaluation of critical components in the aerospace and power generation industries. Many of these critical components must endure extremely high stresses in the course operation. In the gas turbine engine industry, increases in thrust to weight ratios and increases in duration between inspections require even greater durability to assure reliable engine operation. It is necessary to detect even small flaws in order to ensure the durability of the component. For example, a rotor disk for a gas turbine engine must have its entire surface inspected in order to detect the presence of defects. An inability to detect flaws of a certain size can prevent production of higher performance and more competitive products. Moreover, the surfaces on these critical components often have curves, corners, and/or irregular shapes, i.e. complex geometry, maling them even more difficult to inspect.
An eddy current probe typically includes a driver coil and a receiver coil. When provided with an electrical excitation current, the driver coil generates an alternating electromagnetic magnetic field that results in a magnetic field in a component under inspection, which in turn results in an eddy current within the component. The eddy current in the component results in a electromagnetic signal or response, within the receiver coil, detected by commercial instrumentation. As the probe passes over an anomaly, e.g., a flaw or a different morphology, in the component, the anomaly disrupts the eddy current, thereby resulting in a different signal within the receiver coil. The change is detected by the instrumentation.
Two criteria commonly used for appraising an eddy current probe include the sensitivity of its response, and the uniformity of its response as measured at different points along the width of the probe, referred to herein as uniformity. Sensitivity is a qualitative measure that indicates the capability of the probe to detect flaws of a particular size. For a probe to be useful for an application, it must have sufficient sensitivity to detect flaw sizes of interest. Uniformity is an indication of the useful width of the probe. Probes do not have the same sensitivity at all points across the width of the probe. The sensitivity at the edge is typically lower than at the center and may not be suitable for the application. A greater useful width, in effect means that the probe inspects a wider area at one time, referred to as wider field coverage, enabling a more rapid overall inspection. Uniformity is also an indication of the usefulness of the probe for inspecting components having complex geometries, for example those having corners. Typically, the edge is the only part of the probe that can be positioned near a corner. If the probe has low sensitivity at its edges, the probe will not be able to detect a flaw near the corner.
Traditionally, the only eddy current probes having suitable sensitivity for use in inspecting a rotor disk have had very small probe elements, e.g., 0.030 to 0.060 inches wide. The use of probes with such small elements greatly increases the time and cost required for inspection. Even with fully automated scanning systems, the inspection of a single rotor disk currently takes over 80 hours due to the narrow useful width, i.e., narrow field coverage, of these small probes.
Eddy current probes capable of inspecting a wide area on a component, i.e., wide field coverage, are known. Such probes, commonly referred to as wide field eddy current probes, are wider than an ordinary eddy current probe. Greater width provides the probe with greater surface area to thereby inspect a wider surface area on the component. However, as a result of the greater width and surface area, present day wide field eddy current probes have insufficient sensitivity for inspection of many critical components in aerospace and power generation industries. Furthermore most wide field eddy current probes do not provide a sufficiently uniform response, the sensitivity of the probe decreases excessively near the edges. As such, they are not well adapted to inspecting surfaces with complex geometries, e.g., those having corners.
U.S. Pat. Nos. 5,442,286 to Sutton, Jr. et al., and U.S. Pat. No. 5,262,722 to Hedengren et al. disclose eddy current probes having eddy current probe elements disposed within thin multi-layer structures. The driver and the receiver are disposed on adjacent layers in the structure. Such probes are well adapted to inspecting complex geometries, but they often have less than desired sensitivity, e.g., low signal to noise ratio.
Another type of eddy current probe is referred to as an electric current perturbation probe. In a perturbation probe, the driver core axis is perpendicular to the receiver core axis. This feature decouples the receiver magnetic field from the driver magnetic field, thereby reducing the sensitivity of the receiver to surface noise that does not represent a defect. Some materials, for example titanium, present more surface noise than others. Perturbation probes typically provide high sensitivity but do not provide wide field coverage and are not adapted to inspecting complex geometries.
Hoshikaw et al. ("A NEW ECT PROBE WITH ROTATING DIRECTION EDDY CURRENT", Koyama et al,. Review of Progress in Quantitative Nondestructive Evaluation, Vol. 15; "BASIC STUDY OF A NEW ECT PROBE USING UNIFORM ROTATING DIRECTION EDDY CURRENT", Koyama et al., Vol. 16) disclose one type of perturbation probe referred to as an eddy current tester and probe employing a rotating direction eddy current. The probe employs a driver having a cube shape, 30 mm on a side, with two orthogonal windings, and a pancake receiver. Hoshikaw et al. discloses that the probe generates a large amount of data on a flaw with minimal noise. However, this probe does not provide a wide inspection field, nor is it adapted to inspecting complex geometries.
U.S. Pat. No. 5,483,160 to Gulliver et al. disclose a multi-sensor probe having a driver coil and one or more, e.g. four, perturbation coils, i.e. receiver coils with axes perpendicular to that of the driver coil. The perturbation coils each have a flat shape and are mounted on adjacent facets of a probe head. The perturbation coils are longer (dimension parallel to direction of movement of probe relative to component) than they are wide (dimension perpendicular to direction of movement of probe relative to component). However, this probe is not a wide field probe nor is it likely to have suitable sensitivity for use in inspecting critical components.